1. Field of Invention
The present invention relates generally to a method and apparatus for selecting a Modulation and Coding Selection (MCS) level in a wireless mobile communication system.
2. Description of the Related Art
A typical broadband wireless mobile communication system uses Adaptive Modulation and Coding (AMC) in order to increase the data transmission efficiency of a wireless link and uses Outer Loop Rate Control (OLRC) in order to increase a Modulation order Product code Rate (MPR).
The AMC ensures high data transmission performance by differentiating modulation and coding methods according to a wireless channel condition and wireless channel fluctuation of a terminal. The AMC applies different modulation and coding methods between terminals in a short range from and terminals distant from a base station. The OLRC selects an MCS (also referred to as MPR) level for data to be transmitted, according to whether Hybrid Automatic Repeat reQuest (HARQ) information (that is, a HARQ ACK signal or a HARQ NACK signal) is received from a MAC terminal and reflects the selected MCS level to scheduling.
A terminal reports a downlink Channel Quality Indicator (CQI) and HARQ feedback (HARQ ACK or HARQ NACK) information to a base station at regular time intervals. Then, the base station selects an MCS level for a data block to be transmitted based on the received feedback information and reflects the selected MCS level to scheduling.
The base station transmits data with a higher MCS level as receiving a higher CQI from the terminal and with a lower MCS level as receiving a lower CQI from the terminal. Also, the base station increases or decreases an MPR in response to an HARQ ACK signal or an HARQ NACK signal. Generally, the base station increases an MPR if ACK signals are successively received one or more times and decreases an MPR if NACK signals are successively received one or more times. Increasing an MPR means increasing an MCS level, and decreasing an MPR means decreasing an MCS level.
However, if a change of an MCS level by MAC operation is different from a change in wireless environment that a terminal actually experiences, frequency band efficiency may be reduced. Accordingly, a method for overcoming the problem is needed.
Furthermore, in the OLRC, a range for MPR adjustment according to HARQ feedback is fixed, which disables adaptive operation according to a channel environment and a terminal's condition. If a narrow variable range of MPR is set, it is difficult to quickly respond when a terminal moves or an environment, such as a fading channel environment, in which channel fluctuation is significant. In contrast, if a wide variable range of MPR is set, frequency band use efficiency is reduced in a stable channel environment such as Additive white Gaussian Noise (AWGN), resulting in degradation of data transmission performance, although a quick response to an environment in which channel fluctuation is significant is possible.
The above information is presented as background information to assist with an understanding of the present invention. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.